Thermistors are temperature sensitive electrical resistors which can be used as thermometers, temperature sensors, and temperature probes over a wide range of temperatures. Some thermistors may operate at extreme high or low temperatures exhibiting a high degree of temperature sensitivity over a specified range of temperatures. Others may be less sensitive or less able to discriminate fractional degrees of temperature, but may be operable over an even wider range of temperatures.
Most thermistors are constructed from polycrystalline metal oxide materials. These thermistors have can be formulated for wide-ranging temperature measurement and control applications. They can be small, highly sensitive and low cost. However, generally speaking, their reproducibility is poor. See Encyclopedia of Physics, 2nd Ed., Lerner and Trigg, VCH Publications, 1990, p. 1275. Monocrystalline materials, when and if available, may offer a higher degree of uniformity in terms of their physical properties, as well as a high degree of repeatability and efficiency when compared to devices made from equivalent polycrystals.
Monocrystals, as their name implies, are single crystals having highly uniform properties throughout. It is believed that the desirable properties of monocrystalline thermistors relate, at least in part, to certain characteristics of the monocrystalline state. Because monocrystals have no grain boundaries or inclusions, they will not suffer from, for example, charge carrier scattering. Further, although the behavior of any material is not predictable, because of their highly oriented structure, monocrystals are more likely to have consistent properties from one sensor to the next. Finally, when exposed to environmental conditions and changes in temperature, sensors made from monocrystals will not suffer from the accelerated aging exhibited by polycrystalline devices. Polycrystals have voids and other irregularities which can adversely affect their properties or, at very least, render their properties less reproducible from one sensor to the next. Moreover, because of the voids and irregularities inherent therein, polycrystalline based devices can absorb moisture. Therefore, aging can be accelerated when the thermistor is subjected to changing temperatures. Monocrystals could be electroded and used as sensors. However, growing single crystals of functionally equivalent size and electrical properties would be difficult in any type of mass production setting. Therefore, while monocrystals are an attractive alternative to polycrystals in some ways, actual concerns may limit the practical advantages realized.
In an unrelated art, computer chips and integrated circuits are manufactured in a variety of ways from a variety of materials. Generally, a wafer of semiconducting material is cut from an ingot, polished and then patterned with integrated circuitry and the like. The semiconducting wafer is usually made from either elements which occupy Column IV on the Periodic Table such as Si and Ge or combinations of elements in Periodic Table Columns which are an equal distance to either side of Column IV. For Example, the Column III element Ga plus the Column V element As yields a III-V semiconductor GaAs. Similarly, the Column II element Zn plus the Column VI element O yields the III-VI semiconductor ZnO. This relationship is related to the chemical bonding in semiconductors, where, on the average, there are four valence state electrons per atom. See Pierret, "MODULAR SERIES ON SOLID STATE DEVICES, Semiconductor Fundamentals", Vol. I, pages 3-15, Addison-Wesley Publishing Company (1983), the text of this reference, are hereby incorporated by reference.
Of course, in the semiconductor industry, it is a significant disadvantage that the substrate would have electrical properties that vary with temperature. In fact, if possible, those in the industry would attempt to eliminate such temperature based electrical variation as it is considered a significant disadvantage. Moreover, in producing semiconductors using this type of wafer technology, there are several factors which influence the size of the resulting material. However, the types of electrical properties important to temperature measuring are not foremost among those factors. Instead, substrates are prized for homogeneity, structural integrity and consistency of electrical properties, as well as their ability to interact with dopants or electrical circuits imprinted thereon. There are instances of using elemental materials such as carbon, germanium or silicon for temperature sensors. See Encyclopedia of Physics, 2nd Ed., Lerner and Trigg, VCH Publications, 1990, p. 1275. However, to the best of current knowledge, these sensors are composed of polycrystals.